The present invention relates generally to fullerenes and more particularly to fullerenes having a small bandgap. Still more particularly, the present invention relates to small bandgap fullerenes and both small bandgap and radical (zero bandgap) endohedral metallofullerenes, and a method and system for isolating and recovering these molecules.
Fullerenes are the class of carbon compounds distinguished by their multi-faceted, closed molecular structure. The nature of the electron xe2x80x9cshells,xe2x80x9d or orbitals, that surround the nucleus of every atom dictates that each orbital is xe2x80x9cfilledxe2x80x9d when it contains a certain number of electrons. Atoms bond to form molecules because by bonding they can share electrons and fill shells that would otherwise be only partially filled. An unbonded carbon atom has four electrons in its outermost shell, but would prefer to have eight. For this reason, carbon atoms bond readily with other atoms, including other carbon atoms.
Under certain conditions, carbon atoms bond together such that the carbon-carbon bonds form a framework of hexagons and pentagons that resembles the familiar hexagon/pentagon surface of a soccer ball. Molecules having this structure have come to be known as fullerenes. The number and positioning of the hexagons and pentagons can vary, within both constraints that exactly 12 pentagons and that an even number of carbon atoms be present It happens that the spherical molecule formed by sixty carbon atoms (C60) comprises a parfioularly stable combination of hexagons and pentagons and is the most widely studied fullerene to date. In general, more than one arrangement of the hexagons and pentagons is possible, leading to a great variety of possible isomers for any particular number of carbon atoms in a fullerene. To help specify a particular fullerene isomer, the symmetry group name to which that isomer belongs is affixed to the molecular formula, but even this is imperfect as it is common for many isomers belonging to the same point group to be present for any particular number of carbon atoms.
The currently known methods for making fullerenes involve evaporating carbon atoms and cooling them slowly, so that some of them assemble into fullerene molecules. Even under optimal conditions, however, not all of the evaporated carbon atoms end up in fullerene molecules (the remainder forms soot). While C60-Ih forms a significant fraction of the total fullerene production, the fullerene molecules that are produced can have more than three hundred carbon atoms. Under current practices, the fullerenes are extracted from the soot using a non-polar solvent, such as toluene. C60-Ih dissolves readily in such solvents, as do several other fullerenes with isolated pentagons, including but not limited to, C70-D5h, C76-D2, C78-C2vxe2x80x2, C78-C2vxe2x80x3, C78-D3, C80-D2, C84-D2d and C84-D2. Once dissolved, the fullerenes can be recovered in a relatively pure form. Additionally, there are many empty fullerene structures that are predicted to be stable, but which are not found among the fullerenes that are extracted using the method described above. These include, but are not limited to, isolated pentagon isomers of C74 (D3h) and many larger fullerenes, such as C78-D3hxe2x80x2 and C80-Ih.
Fullerenes can also be produced with one or more atoms of another material trapped inside the cage formed by the fullerene molecule. When the trapped atom is a metal, the molecule may be called a metallofullerene or endohedral metallofullerene. While various attempts have been made to produce endohedral metallofullerenes, with one exception, the only endohedral metallofullerenes that have been recovered are those containing metal atoms that have an even total number of valence electrons. For example, endohedral metallofullerenes containing Group II metals (calcium, strontium, and barium) have been isolated. Also, endohedral metallofullerenes containing two Group m metals (scandium, yttrium, and the lanthanides) have been recovered. However, endohedral metallofullerenes containing a metal atom(s) that has an odd number of valence electrons (one or three Group III atoms, e.g.) are, in general, not recoverable. The aforementioned exception occurs when the fullerene cage has 82 carbon atoms. Because they have never been recovered or isolated, the very existence of other endohedral metallofullerenes as stable, recoverable molecules has not been considered certain.
Hence, it is desirable to provide a method for recovering these previously unrecoverable fullerenes and metallofullerenes. It is further desired to provide a fullerene isolation method that is simple and easy to execute, and that does not disrupt or affect the subject fullerenes.
The present invention provides a method for recovering previously unrecoverable fullerenes. The present method is simple and easy to execute, and does not disrupt or affect the subject fullerenes. According to one embodiment of the present invention, fullerenes in question are reduced through the addition of electrons to a charged state in which they can be dissolved in certain solvents. Once dissolved, the fullerenes can be recovered by removing the charge and returning them to their charge-neutral state.
According to a preferred embodiment, the subject fullerenes are recovered by providing them with an electronic charge sufficient to overcome their tendency to fill their electron-deficient orbitals by forming intermolecular bonds. Once their bonds have broken and they have gone into solution as anions, the subject fullerenes can be recovered by oxidation, i.e. by removing the added charge. The preferred isolation technique includes, but does not require, additional processing steps such as sublimation of the fullerenes to separate them from the bulk of the non-fullerene soot, and the preliminary removal of known fullerenes from the starting material using a known fullerene solvent.